Don't allow consecutive same character and some different character together in TextFormField

I want to restrict the TextFormField to only accept numbers separated by commas and sometimes with dashes but I don’t want them to come consecutive to each other and also don’t want the same character consecutive.

Ex:-

• 1,3-4,9-11 is correct
• 1,,3--4,9-11 is wrong
• 1,-3-4,9-11 is wrong
• 1-,3-4,9-11 is wrong

To restrict things to only numbers, commas and dashes I’m using:-

FilteringTextInputFormatter(
   RegExp("[0-9,-]"),
   allow: true
)

But it is not restricting the consecutive behavior as shown in the wrong behavior in the examples.

So, how can I restrict my TextFormField to the correct behavior represented in the examples?

Thank you.

>Solution :

If you want to validate on submit, you might write the pattern as:

^[0-9]+(?:[,-][0-9]+)*$

Regex demo

If a negative lookahead is supported, you an exclude matching 2 times one of - or , while validating on typing.

Note that this will allow , or - at the end:

^(?!.*[,-][,-])[0-9,-]*

Regex demo